This is an experiment: by using pip's internal vendored distlib, we can
generate script entry points for Windows, Mac and Linux using distlib's
mechanisms. This is the same mechanism used when running "pip install",
so it will be consistent with the most common method of package
installation on all platforms. It also allows us to delete a good bit of
vendored/borrowed code from inside of mkvenv.py, so there's less to
maintain and the license might be more straightforward.
As a downside, if we're not willing to actually add a distlib
requirement, we have to piggy-back on pip's vendored version, which
could have downsides if they move our cheese in the future.